Even stabilising the world's climate at two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels -- the daunting goal laid down in the 196-nation Paris Agreement -- would melt more than 40 percent of permafrost, or an area nearly twice the size of India, they reported in the journal Nature Climate Change.

That could take centuries or longer, but would eventually drive up global temperatures even further as more gases escaped into the air.

Sometimes called a climate change time bomb, the northern hemisphere's 15 million square kilometres (six million square miles) of increasingly misnamed permafrost contains roughly twice as much carbon -- mainly in the form of methane and carbon dioxide (CO2) -- as Earth's atmosphere.

Currently, the atmosphere holds about 400 parts per million of CO2, 30 percent more than when warming caused by human activity started in the mid-19th century.

"We estimate that four million square kilometres (1.5 million sq. miles) -- give or take a million -- will disappear for every additional degree of warming," said co-author Sebastian Westermann, a senior lecturer at the University of Oslo.

Human-induced global warming has already caused the planet to heat up by 1C (1.8F), and is on track to add at least another 2C (3.6F) by century's end unless global emissions are slashed in the coming decades, the UN's climate science panel has concluded.

- Back to basics -

Those calculations do not include the possible impact of melting permafrost.

The most recent report from the UN's Intergovernmental Panel for Climate Change (IPCC) "talks mainly about the uncertainties," and discounts the likelihood that gases released from melting soils will significantly add to warming by 2100.

But climate models -- which vary depending on predicted levels of greenhouse gas emissions -- are all over the map in forecasting the future of permafrost.

To sidestep some of these uncertainties, a team of scientists led by Sandra Chadburn of the University of Leeds used a "back to basics" approach based on observations.

"Our method allows for a projection of how much permafrost will be lost at what temperature -- but it doesn't tell us how long that will take," said Westermann.

The findings should serve as a benchmark for future climate change models, he added.

"If climate models show something very different, scientists will have to explain why it is not in agreement with the observations."

Permafrost is found in a wide belt between the Arctic circle to the north and boreal forests to the south, across northern Europe, Russia, Alaska and Canada.

It can vary in depth from a few metres to more than 100, but most carbon stocks are thought to reside fairly close to the surface.

Roughly 35 million people live in the permafrost zone, some in large cities where buildings risk collapsing in the next two decades as the ground softens.